The constantly increasing demand for energy and climate change must be met by the use of sustained energy sources. Solar energy is such a sustained energy source. It is climate-friendly, available in inexhaustible quantities, and does not represent a burden for subsequent generations.
Solar-thermal power plants represent an alternative to conventional power generation. At the present time, solar-thermal power plants are operated with parabolic trough collectors and indirect evaporation. Direct evaporation in parabolic trough collectors or Fresnel collectors represents a future option. A solar-thermal power plant with parabolic trough collectors or Fresnel collectors and direct evaporation consists of a solar field in which the supply water is preheated, evaporated, and superheated, and of a conventional power plant part, in which the thermal energy of the steam is converted into electrical energy.
In non-stationary operation (e.g. load change), with forced-flow parabolic trough collectors or Fresnel collectors with direct evaporation, the evaporator flow must be changed as synchronously as possible into the heat input into the evaporator heating surface. The nominal value control of the supply water flow regulation provides the necessary supply water nominal values as a function of the state of the installation, in start-up and low-load operation as well as in forced-flow operation. With non-stationary behavior of the solar field it has the task of guaranteeing a desired evaporator output enthalpy on the flow medium side, and therefore impeding associated, in particular substantial, temperature fluctuations of the steam at the evaporator output, with all the overlaid effects incurred (e.g. live steam temperature fluctuations).
Changes in the heat input into the evaporator heating surface of the solar field and/or interferences in the evaporator input enthalpy, at a given flow rate, have a direct effect on the evaporator output enthalpy. An adjustment of the evaporator flow rate can only be carried out, in the fastest case, after a control deviation, which under certain circumstances, specifically for rapid load transient events (e.g. cloud passage), can be too sluggish to guarantee a fluctuation-free output temperature. Under these conditions, the regulation basically limps after the change in the solar-side heat supply. Serious disturbances in the thermodynamic state values (in particular, high temperature fluctuations in the water-steam circuit of the solar field) result.
In modern parabolic trough power plants with direct evaporation, the evaporator is overfed. By means of an appropriate apparatus (water-steam separator), the surplus water at the evaporator output which has not yet been evaporated is separated from the steam. The steam flows into the downstream superheater collectors. The surplus water is either collected in the separator itself or in a downstream container (water collection vessel), then, in the further course of the process, is discharged through a purge hardware, and, in the best-case scenario, is mixed back into the main flow again at the evaporator input (recycled).
Under these preconditions, in order to control the necessary evaporator throughflow, use is usually made of what is referred to as a three-component control system, which, as a function of the generated steam mass flow, in the best-case scenario refills precisely the same quantity of supply water. A correction regulator which, for example, regulates the water level in the water collection vessel, corrects the supply water quantity determined in this way in the event of the actual water level deviating from the specified nominal value (e.g. in the event of dynamic processes and in order to take account of necessary discharge mass flows during the purging process).
The advantage of this method lies in the low-fluctuation medium temperature at the evaporator output, since this corresponds to the saturation temperature. In addition to this, in comparison with a continuous concept with which, as a rule, superheated flow medium is present at the evaporator output, it is highly probable that a more stable flow form can be achieved, even if no additional flow-stabilizing measures are taken with regard to the continuous concept. However, because the evaporation end point is spatially fixed at the evaporator output, the advantage is lost of the operational flexibility of a forced-flow steam generator with variable evaporation end point, such as, for example, guaranteeing the most constant possible live steam temperatures over a wide load range. Under these circumstances, the demands on the live steam temperature control system increase. In addition, a reasonable control of the water level in the water collection vessel, specifically for rapid load transient events, can only be achieved with difficulty, or even not at all, due to the low volume of the water collection vessel and the long time delay behavior of the controlled system.